This invention relates to capacitive transducers, and more particularly to variable capacitance transducers for determining the level of fluent materials within a container.
Transducers for determining liquid level are often used in vehicles, industrial equipment and other systems and components. Such transducers typically operate by detecting a change in an electrical property of the transducer which varies in accordance with the liquid level.
By way of example, prior art liquid level sensors, such as fuel sensors for motor vehicles, usually include a float that rides on an upper surface of the fuel in a fuel tank. The float is typically connected to one end of a pivot arm while the other end of the pivot arm typically includes a wiper mechanism that brushes against a resistor strip when the arm is rotated due to a change in fuel level in the tank. Such sensors are prone to wear, mechanical and/or electrical breakdown or inaccurate liquid level detection. Although variable capacitance probes have been developed to overcome these drawbacks, they are cost-prohibitive in many applications and are typically limited to measure a certain type of liquid, since different liquids will have different dielectric properties.
In addition, a variable capacitance probe designed to measure fuel level normally cannot be used for measuring water level due to the different dielectric properties associated with different liquids. For example, the dielectric constant at room temperature of a vacuum is one, of air is close to one, of gasoline is about two, of industrial alcohol is anywhere from 16-31, and of water is about 80. Since capacitance is directly dependent on the dielectric constant, a transducer designed for measuring the level of one type of liquid could not be relied upon for measuring other types of liquids. However, even when the transducer is designed for measuring only one type of liquid, such as gasoline, the dielectric constant can change due to different gasoline formulations, the presence of water, alcohol, detergents, additives, as well as environmental factors such as temperature, thus leading to significant measurement inaccuracies.
In order to overcome these challenges, the prior art has suggested compensation means in the form of a reference capacitor at the bottom of the tank, which must always be immersed in the liquid being measured. The reference capacitor includes a pair of spaced plates and the liquid being measured serves as the dielectric between the plates. A dielectric constant of the liquid can then be determined and used to offset the capacitance of the liquid level sensor to compensate for dielectric variations. However, this type of solution only takes into account changes in dielectric at the bottom of the tank since it is assumed that the liquid is uniform throughout it's volume. In reality, since some liquids have a greater density than others, there may be a separation of fluids at different depths that cannot be discerned by a single reference capacitor. For example, the reference capacitor may be exposed to water or other contaminants at the bottom of a fuel tank which may lead to inaccurate dielectric compensation.
In addition, the electronics associated with capacitive measurement and compensation are relatively expensive and are thus priced out of markets where there is a long-felt need for low-cost and highly accurate liquid level transducers.